1. Field of the Invention
This invention relates to a semiconductor laser device used in transmission devices, etc. of the optical communication systems.
2. Related Background Art
The semiconductor laser device used in the optical communication using optical fibers is exemplified by the one as shown in FIG. 1 (a first prior art) which comprises a semiconductor laser chip 1A as a signal light source and a photodiode chip 1B as light detecting means for monitoring the light emitted from the rear end surface (lower end surface as viewed in FIG. 1) of the semiconductor laser chip 1A which is mounted on a header 2, and lead wires 3 connecting the photodiode chip 1B to a monitor circuit and connecting the semiconductor laser chip to an outside drive circuit (not shown).
In FIG. 1, a cap 4 has a light transmitting window 5 which passes the signal light emitted from the semiconductor laser chip 1A and airtightly seals the semiconductor laser chip 1A and the photodiode chip 1B.
A condenser lens 9 secured to a lens holder 10 is used to cause the signal light emitted from the semiconductor laser chip 1A to effectively enter an optical fiber 6 inserted in a housing 8 through a ferrule 7. Especially the structure of FIG. 1 is called coaxial type.
On the other hand, the semiconductor laser device (a second prior art) shown in FIG. 2 is called butterfly type and has a housing 8 of rectangular section. This semiconductor laser device has basically the same function as the first prior art. But it is possible that an IC (integrated circuit) chip for the laser drive circuit and the monitor circuit is mounted on the header 2 secured in the housing 8.
The photodiode chip 1B as the light detecting means comprises, as shown in FIG. 3, an epitaxial layer (semiconductive crystal layer) 12 grown on a semiconductor substrate 11, and a diffusion region 13 formed on the surface of the epitaxial layer 12 by thermal diffusion of a metal element and having an opposite polarity to that of the epitaxial layer 12.
The interface between the epitaxial layer 12 and the diffusion region (first region) 13 have a pn junction, so that the light radiated to the pn junction contributes to the generation of a photocurrent. The generated photocurrent is taken outside for monitor through electrodes 14, 17.
FIGS. 4 and 5 respectively show a top view and an X--X sectional view of a structure of the photodiode chip 1B described above ( FIG. 3 ). As shown, in the photodiode chip 1B, a semiconductive crystal layer 12 of a first conductivity type including a light absorption layer is laminated on a surface of a semiconductor substrate 11 of a first conductivity type having an electrode 17 of the first conductivity type formed on an underside there, and impurities are selectively diffused into the semiconductive crystal layer 12 to form a first region 13 of the second conductivity type. This is a pin photodiode structure where the semiconductor substrate 11 is an n layer (or a p layer), the semiconductive crystal layer 12 is an i layer and the first region 13 is a p layer (or an n layer), and a photo-sensing region is formed in the i layer. An electrode 14 of a second conductivity type is formed on the first region 13 on the surface of the semiconductor crystal layer 12, and the first region 13 inside the electrode 14 is covered with an anti-reflection film 16 while the semiconductive crystal layer 12 outside the electrode 14 is covered with a protection film (i.e. passivation film) 15.
When a reverse bias is applied to the semiconductor device thus constructed, a depletion layer is created in a pn junction area in the semiconductive crystal layer 12. Thus, an electric field is developed in the depletion layer and electrons and holes generated by a light applied to the photo-sensing region 18 are directed to the first conductivity type semiconductor substrate 11 and the second conductivity type region 13, respectively, and accelerated thereby. In this manner, a photocurrent is taken out and a light signal is detected.
In the structure shown in FIGS. 4 and 5, when the light is applied to the photo-sensing region 18, light generating carriers are captured by the depletion layer and a good response characteristic is offered. However, when the light is directed to the outside of the region 18, the generated carriers reach the pn junction while they are diffused by a density gradient and are taken out as a photocurrent. As a result, the response characteristic is adversely affected. FIG. 6 shows a response characteristic of the photodiode chip 1B. Since the movement of the carriers by the diffusion is slow, a response waveform for a light pulse includes a tail at the end as shown in FIG. 6.
When such a photodiode chip 1B is used for the light communication, a light emitted from an optical fiber is condensed so that it is directed to the photo-sensing region 18. However, when a portion of light leaks out of the photo-sensing region 18, it leads to the reduction of the response speed of the photodiode chip 1B by the reason described above. In a high speed photodiode chip, the area of the photo-sensing region 18 is reduced to reduce a junction capacitance. As a result, a ratio of light directed to the outside of the photo-sensing region 18 increases and a diffused component which has a low response speed increases. This leads to the degradation of the response speed.
When the light emitted from a rear end plane of the semiconductor laser device is sensed by the photodiode chip 1B to feedback-control a drive current for the semiconductor laser device in order to keep the light output of the semiconductor laser device at a constant level, if the light emitted from the semiconductor laser spreads to the outside of the photo-sensing region 18 of the photodiode chip 1B, a low response speed component is generated by the diffusion as described above. This adversely affects to the feedback control.
These conventional semiconductor laser devices have the following problems.
The above-described semiconductor laser devices have properties that the semiconductor laser chip 1A as the signal light source tends to have unstable emitted light intensities corresponding to temperature changes. The signal light emitted from the semiconductor laser chip 1A is monitored by the photodiode chip 1B to control an average current to be applied to the semiconductor laser chip 1A to be constant so that a level of the light monitored by the outside electronic circuit is maintained constant.
But to operate the semiconductor laser chip 1A more stably at a high speed above about 100 Mbps, the control of the average current is not sufficient. Actually it is also necessary to detect a minimum emitted light intensity and a maximum one and control a level of the emitted light intensity. But the conventional semiconductor laser devices, which use the photodiode chip 1B of the above-described common structure, cannot accurately detect a minimum and a maximum emitted light intensity, and it is difficult to put the conventional semiconductor laser devices to practical uses.
This is because a photocurrent generated by the light radiated to a part of the surface of the photodiode apart from the light detecting region (pn junction) becomes a slow response component which does not follow a change of an emitted light intensity of a laser beam.
In FIG. 3 reference numeral indicates a divergence of a light beam emitted from the semiconductor laser chip 1A. Carriers generated by those of the beams absorbed in the light detecting region 13 (a light detecting window) on the surface of the photodiode chip 1B and its very neighboring area (e.g., 3.about.5 .mu.m) are effectively isolated at high speed by an electric field applied to the pn junction and contribute to the generation of a photocurrent. But the signal light absorbed outside the region and the area adversely generate a very slow response photocurrent because the electric field is not applied thereto.
Consequently, in monitoring digital signal light, signal waveform distortions occur (especially a rectangular pulse trails by .mu.sec at the fall), and a maximum and a minimum values cannot be accurately detected. High-speed, stable modulation cannot be performed.
In the conventional semiconductor laser devices, because of this problem, a position of the photodiode chip 1B on the header is finely adjusted so that the signal light emitted from the rear end surface (the lower end surface in FIG. 1) is incident only on the light detecting region of the photodiode chip 1B. A resultant problem is that it takes more time to mount a photodiode chip 1B on the header 2 because the mounting position of the photodiode chip 1B on the header 2 has to be finely adjusted. This problem is a neck to the massproduction.
Although the semiconductor laser device is essential to the optical communication, the device has such problems and cannot be put to all uses, which is a neck to the rapid prevalence of the optical communication systems.